Utilizing Water Emulsification to Reduce NOx and Particulate Emissions Associated with Biodiesel

Abstract

A key barrier limiting extended utilization of biodiesel is higher NOx emissions compared to petrodiesel fuels. The reason for this effect is unclear, but various researchers have attributed this phenomena to the higher liquid bulk modulus associated with biodiesel and the additional heat released during the breaking of C-C double bonds in the methyl ester groups. In this study, water was incorporated into neat biodiesel (B100) as an emulsion in an attempt to lower NOx and particulate matter (PM) emissions. A biodiesel emulsion containing 10wt% water was formulated and evaluated against an ultra-low-sulfur petroleum diesel (ULSD) and neat biodiesel (B100) in a light-duty diesel engine operated at 1500 rpm and at loads of 68 and 102 Nm (50 and 75 ft-lbs). The influence of exhaust gas recirculation (EGR) was also examined. The incorporation of water was found to significantly lower the NOx emissions of B100 while maintaining fuel efficiency when operating at 0% and 27% EGR; however, NOx emissions were observed to increase slightly for the emulsified fuel when the engine load was raised to 102 Nm (75 ft-lbs). The soot fraction of the particulates (as determined using an opacity meter) was much lower for the B100 and B100-water emulsion compared to the ULSD. In contrast, total PM mass (for the three fuel types) was unchanged for the 0% EGR condition but was significantly lower for the B100 and B100-emulsion during the 27% EGR condition compared to the ULSD. Analysis of the emissions and heat release data indicate that water enhances air-fuel premixing to maintain fuel economy and lower soot formation. The exhaust chemistry of the biodiesel base fuels (B100 and water-emulsified B100) was found to be unique in that they contained measurable levels of methyl alkenoates, which were not found for the ULSD. These compounds were formed by the partial cracking of the methyl ester groups during combustion.